The present invention relates to an X-ray CT apparatus and an X-ray distribution adjusting filter apparatus, known as a bowtie filter, for use in X-ray CT apparatus.
An X-ray source used in an X-ray CT apparatus outputs X-rays having a certain energy width. The linear absorption coefficient of X-rays penetrating the subject is dependent on the X-ray energy, and there is manifested a beam hardening (BH) effect, which means that the greater the penetration length of the subject, the higher the average energy tends to be. Therefore, the penetration intensity of X-rays, i.e. the value of projected information to generate signals detected by an X-ray detector in the X-ray CT apparatus, is not in proportion, but in a nonlinear relationship, to the penetration length.
Since the BH effect invites the cupping effect of inviting a drop in intensity in the central part of the reconstructed image on an X-ray CT apparatus, the detection signals from the X-ray detector have to be corrected, and this correction is accomplished by figuring out for each channel of the X-ray detector a correction coefficient for the value of projected information to generate a reconstructed image of uniform intensity.
For correction at a higher level of accuracy, a phantom is used. As such a phantom, a cylindrical phantom whose plurality of sections have different diameters large enough to cover substantially the whole field of view (FOV), arranged at the center of the image, is picked up, and projected information from these sections of the phantom is used to increase the accuracy of correction using the correction coefficient.
For such X-rays which diffuse (disperse) as they deviate from the center axis between the X-ray source and the X-ray detector, in order to uniformize the intensity of X-rays penetrating the subject (or the phantom) or to adjust the profile of the X-rays, the X-ray emitting section of the X-ray source is provided with an X-ray distribution adjusting filter apparatus, known as a bowtie filter, and the X-ray intensity is thereby made as uniform as possible independent of the distance from the center axis.
[Patent Reference 1]
Japanese Patent Application Laid-open No. 2002-102217
However, the sectional shape of the subject region positioned in the FOV may greatly differ from one subject to another. For instance, even of the same subject, the sectional shape greatly differs among the head, chest and trunk of the body. Of course, adults and children entirely differ in overall sectional shape.
Therefore, even though the X-ray intensity distribution is adjusted by using an X-ray distribution adjusting filter apparatus, known as a bowtie filter, appropriate adjustment is further desired to take account of the difference in the overall size of the subject and differences among regions of the same subject.
Especially in recent years, even more precise tomography with an X-ray CT apparatus has come to be required, and appropriate adjustment of X-ray distribution differentiated according to the examined (imaged) region of the subject is needed along with accurate channel-by-channel correction of projected information value using the aforementioned phantom.